According to a known embodiment, a part made of a composite can be made from plies of prepreg resin fibres. In a first phase called lay-up, the fibre plies are placed in a mould comprising, on the one hand, a plate for shaping the surface of the part at its contact and, on the other hand, peripheral side pads which shape the sides of the part. After a curing phase for consolidating the part by subjecting it to a temperature and pressure cycle, the part is ejected, and then inspected before being machined to the final dimensions.
During curing, the resin tends to migrate away from the periphery of the part, so that the height of the part tends to decrease from the middle towards the sides thereof. This phenomenom, called the edge effect, gives rise to the production, after curing, of a part 10 as shown in FIG. 1.
This part, presumed to have a rectangular cross section with the final dimensions, comprises, after curing, a rectangular central portion 12 with, at the periphery, edges with a face 14 inclined to the central portion, so that the sides of the part have a lower height than that of the central portion. Thus, the part obtained after curing comprises peripheral zones 16 having a trapezoidal cross section having a width X that may vary according to the thickness of the part and the materials.
In a first operating procedure, insofar as the part obtained after curing must be machined to the final dimensions, it is possible to take account of the edge effect and to increase the dimensions of the blank so that the width A is smaller than the width X of the zones to be removed at the edges, as shown in FIG. 2. However, this operating procedure is unsatisfactory because it requires increasing the quantity of material used and, more particularly, the quantity of machined material, thereby tending to increase the raw material costs and the production costs, due to the longer lay-up time. Furthermore, it is necessary to modify the tools in order to increase their useful surface areas, in order to maintain a machining time that is substantially equivalent to the time for machining a blank whose dimensions do not take account of the edge effect.
Finally, this operating procedure is contrary to developments in manufacturing processes, which are designed to obtain a blank whose dimensions are as close as possible to those of the finished part.
In another operating procedure shown in FIG. 3, a blank is made whose dimensions do not take account of the edge effect but are close to those of the finished part. After curing and machining, the edges of the finished part are observed to have inclined faces 14. To correct this geometric and/or dimensional defect, at least one additional ply 18 is added at the edge in order to increase the thickness.
In this operating procedure, the method comprises a first lay-up phase, a first curing phase, a first inspection phase, a machining phase, followed by a second lay-up phase in order to deposit the additional plies, a second curing phase for curing the said additional plies in order to consolidate them and to bind them to the rest of the part, followed by a new inspection phase generally associated with a waiver operating procedure.
This operating procedure is not fully satisfactory because it necessarily impacts the part insofar as it undergoes two curing phases. Furthermore, it leads to longer cycle times due to the two curing and inspection phases and to derogation procedures. Thus, in this operating procedure, even if the edge effects can be limited at the geometric and dimensional level, they necessarily influence the structure of the part obtained and on the production cycle times.
Thus, the present invention proposes a method for obtaining a part made of a composite which tends to limit the edge effects at the structural level of the part, while obtaining, after curing, a blank whose dimensions are close to those of the finished part.